On 20 March 2018, the National International Health
Regulations focal point for the Netherlands notified WHO about a human
infected with a new reassortant A(H1N2) of seasonal influenza viruses
that was detected in the routine sentinel influenza surveillance for
influenza-like illness and other acute respiratory infections in the
Netherlands. The patient is a child aged less than two years old, who
had onset of symptoms in early March 2018. He was seen by a general
practitioner, but did not require hospitalization and has fully
recovered.

The general practitioner who attended to this case
participates in the sentinel influenza surveillance network. With
permission from the parent accompanying the child, nose and throat swabs
were collected for virus detection. On 18 March, the Dutch National
Influenza Centre (NIC), which is a collaboration of the Erasmus
University Medical Center in Rotterdam and the National Institute for
Public Health and the Environment (RIVM) in Bilthoven, reported that the
patient was confirmed to be infected by an A(H1N2) influenza virus by
real-time polymerase chain reaction (RT-PCR). Sanger sequencing of the
haemagglutinin (HA) and neuraminidase (NA) genes, and full genome
sequencing revealed that the virus is a reassortant A(H1N2) of seasonal
influenza viruses, and is made up of genes from currently circulating
seasonal influenza virus subtypes A(H1N1)pdm09 (the haemagglutinin [HA]
and the nonstructural [NS] protein genes) and A(H3N2) (the rest of the
genes).

The patient had not travelled abroad, was not vaccinated
against influenza, did not use influenza antivirals and had no
underlying disease.

Further investigations revealed that the parents had symptoms
of (predominantly) gastrointestinal infection in the two weeks preceding
onset of disease in the index. No laboratory testing was performed.
Contact investigation was extended to the day care center attended by
the child before onset of the illness. After the onset of illness, the
index case did not attend daycare anymore. According to a post on
ProMED, extended contact investigation at the child’s daycare did not
reveal any actual disease (1).

Other reassortant A(H1N2) viruses of seasonal A(H1N1) and
A(H3N2) influenza viruses have been detected in the past e.g. between
2000–2003 (2). However the genetic constellations differed from that of
the current virus [i.e. the HA was derived from a seasonal A(H1N1) virus
and the other seven genes from seasonal A(H3N2) viruses].

Public health response

Investigations are still ongoing and enhanced laboratory
monitoring and subsequent subtyping of influenza type A virus positive
specimens in the area where the first reported case lives have been put
in place.

All influenza type A viruses detected in the sentinel
surveillance and viruses submitted to the NIC from non-sentinel sites
(hospital and peripheral laboratories) undergo HA and NA subtyping; no
other detections of similar viruses have been reported. In addition,
influenza type A virus positive specimens from non-sentinel sites in the
region of the patient are being sent to the NIC for subtyping.

The full genome of the reassortant A/Netherlands/10407/2018
A(H1N2) has already been uploaded in the database of Global Initiative
on Sharing All Influenza Data (GISAID).

WHO risk assessment

This is the first time that human infection with a seasonal
reassortant A(H1N2) influenza virus with this particular genetic
constellation has been reported. The patient had relatively mild
disease, did not require hospitalization and has fully recovered.

It is important to note that this reassortant A(H1N2) of
seasonal influenza viruses is different from the variant (v) influenza
viruses found in human cases reported in the United States of America [
A(H1N1)v, A(H1N2)v, A(H3N2)v)] which comprise genes from swine influenza
viruses (3).

Currently, the Netherlands are facing an abnormally long
season of seasonal influenza; it is the 14th week above the epidemic
threshold. Over the last 20 years, the influenza season lasted on
average nine weeks. Influenza B viruses of the Yamagata lineage have
predominated this season but since March 2018, influenza subtypes
A(H1N1)pdm09 and A(H3N2) have been increasingly detected .

WHO assesses the risk posed by this virus to be comparable to
the risk posed by the currently circulating seasonal influenza viruses,
as all the genes of this reassortant virus originate from circulating
seasonal viruses. The hemagglutinin (HA) and nonstructural (NS) genes
are very similar to the genes of the seasonal A(H1N1)pdm09 viruses
circulating recently, and the remaining six genes are very similar to
those of recently circulating A(H3N2) viruses. Further characterization
of the seasonal reassortant A(H1N2) influenza virus is currently
ongoing.

WHO advice

This case does not change the current WHO recommendations on public health measures and surveillance of seasonal influenza.

WHO acknowledges the Dutch sentinel influenza surveillance
system and their active collaboration in information and virus exchange
with the Global Influenza Surveillance and Response System (GISRS) via
the National Influenza Centre (NIC). WHO will notify GISRS to adjust its
laboratory surveillance details as appropriate.

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